1. Field of the Invention
The present invention relates to a method for manufacturing a hybrid printed circuit board.
2. Description of the Related Art
A hybrid printed circuit board refers to a composite printed circuit board wherein electronic parts are mounted on a printed circuit board.
A package wherein electronic parts are electrically connected to the circuit surface of a semiconductor chip via arrayed metal electrodes (bumps) is referred to as flip chip packaging.
A typical flip chip is a ball grid array (BGA) package.
The BGA package is a kind of package boards wherein LSIs, such as a CPU, and other electronic parts, such as capacitors, and they are connected to a printed circuit board (also known as “system board” or “mother board”) by soldering.
The BGA package has a plurality of solder balls (the solder balls are also referred to as “BGA balls” on the joining surface with the printed circuit board.
For solder balls, the use of lead-free solder is required considering the effect on environment.
As shown in FIG. 10A, upon assembling a package module 10, after mounting an LSI 14 and other electronic parts 17 on a package board 12, solder balls 19 are fixed on grid arrayed pads 18.
The LSI 14 is mounted on the package board 12 via solder balls 15, and an under-fill 16 is filled between the solder balls 15 for reinforcing joining force.
The package module 10 is positioned to and loaded on a printed circuit board 20 wherein footprints 22 have been formed.
Thereafter, these are introduced into a reflow furnace (not shown) by a belt conveyor (not shown) and heated.
Thus, as shown FIG. 10B, the solder balls 19 are melted, and soldered to the footprints 22.
Such an assembling process is referred to as a “reflow” process.
Here, FIGS. 10A and 10B are sectional view for illustrating the manufacture of a conventional hybrid printed circuit board 1.
However, the above-described conventional hybrid printed circuit board 1 has disadvantage of poor electrical characteristics.
Specifically, if lead-free solder is used in solder balls 19, the reflow temperature becomes high.
At this time, when the material for the package board 12 is different from the material for the printed circuit board 20, if a difference in coefficients of thermal expansion is large, thermal stress is applied to the joining portion between the solder balls 19 and the footprints 22, and the reliability of joining is lowered.
Since the solder balls 15 are also re-melted during reflowing, if voids due to air bubbles that are produced by pour soldering, the solder balls 19 melt and flow into the voids causing short-circuiting between the solder balls 15.
Furthermore, the adhesive force of the under-fill 16 is lowered by thermal stress, and the solder balls may be peeled off.
The conventional hybrid printed circuit board 1 also has the disadvantage of low manufacturing efficiency.
Specifically, when a conventional hybrid printed circuit board is assembled, an LSI 14 and electronic parts 17 must be mounted on a package board 12 before mounting the package module 10 on the printed circuit board 20.
In other words, all the electronic parts cannot be collectively mounted on the hybrid printed circuit board 1.
Furthermore, since the package module 10 mounted on the printed circuit board 20 is protruded from the printed circuit board 20, it is easily caught by external members to make handling characteristics poor.